pH optimization of gut cellulase and xylanase activities from the eastern subterranean termite, Reticulitermes flavipes (Isoptera: Rhinotermitidae).

摘要

Cellulolytic and xylanolytic termite gut carbohydrolases were assayed in buffers at pH 4.0 to 7.5 for eastern subterranean termite workers, Reticulitermes flavipes (Kollar). The buffer/pH combinations that maximized activity within a gut region were 0.1 M pH 5.5 sodium acetate for endoglucanase and xylanase assays and 0.1 M pH 6.5 sodium phosphate for exoglucanase and beta -glucosidase assays. Endoglucanase activity increased in all gut regions as pH increased from 4 to 5.0-5.5. Highest endoglucanase activity in the foregut was achieved at a pH of 6.5, and foregut activity remained stable as pH increased to 7.5. However, both midgut and hindgut activities were greatest at a pH of 5.5. beta -glucosidase activities in hindgut extract showed the most dramatic increase in activities across pH, increasing about 7 fold between pH 5 and 7. beta -glucosidase activities were negligible at or below pH 5.5. Hindgut exoglucanase activities were consistently greater with increasing pH, following a sigmoid curve with the greatest differences in activity between pH 5.5 and 7 and negligible activity below a pH of 6. Hindgut xylanase activity followed a reverse trend with greatest activity at pH 4.5 and the greatest downward trend from pH 5.5 to 7.5. It appears that crystalline cellulose is more easily digested at elevated pHs, and that the higher pH found in localized areas of higher termites may actually have evolved to enhance the activity of exoglucanases; whereas xylan digestion is optimal in acid environments and could be sacrificed in order to more efficiently digest cellulose.